Wind turbines are normally controlled in accordance with a control strategy based on one or more measured control parameters, such as wind speed, wind direction, etc. Accordingly, a control system used for controlling operation of a wind turbine is normally connected to one or more sensors, each sensor being arranged to measure a specific control parameter. However, most sensors are specified to operate within specific operating ranges, e.g. in terms of temperature, humidity, and some sensors fail or perform incorrect measurement in the case that ice is formed on the sensor. Accordingly, each sensor defines a set of ‘failure modes’ specifying conditions under which the sensor is incapable of operating reliably.
In the case that the control strategy for operating a wind turbine is based on a control parameter measured by a specific sensor, and that sensor is in a ‘failure mode’, then it is not possible to obtain a reliable value for the control parameter until the operating conditions have changed sufficiently to ensure that the sensor is no longer in ‘failure mode’. In the meantime it is not possible to control the operation of the wind turbine based on that specific control parameter, and another, possibly less preferred, control strategy must be used instead. This is undesirably because it inadvertently leads to non-optimum control strategies being used, and thereby to non-optimum energy production.
GB 2 067 247 discloses a wind turbine having pressure probes built into tip portions of the rotor blades for ascertaining the wind energy in the rotor plane. The indications from the pressure probes are used in normal operation of the wind turbine. An anemometer is placed on top of the nacelle and measures wind speed on top of the nacelle. The anemometer is used for control outside the operating range of the turbine, when the turbine is on standstill due to absence of wind or in case of a storm. However, the pressure probes and anemometer do not measure the same control parameter.
DE 20 2006 000816 U1 discloses a wind energy plant being provided with an ultrasonic anemometer and another anemometer, e.g. a cup anemometer, for measuring wind speed. In the case that the wind speed measured by means of the ultrasonic anemometer and the wind speed measured by means of the other anemometer differ from each other by a certain amount a control signal is sent to the control system in order to stop operation of the wind energy plant.